You don't need any 3rd plugin plugins to view source code files. Invoking QuickLook by pressing the spacebar on JavaScript/CSS source code file which is encoded in plain text ASCII will show you the plain source code text.
Here's what QuickLook shows when QuickLooking a JavaScript file named callback.js:
Installing QLColorCode using Homebrew by running brew cask install qlcolorcode and invoking QuickLook on the same file immediately thereafter shows this:
Follow the installation instructions on the project homepage to make sure your installation is correct:
This is a Quick Look plugin that renders source code with syntax highlighting, using the Highlight library.
To install Highlight, download the library manualy, or use homebrew
brew install highlightTo install the plugin, just drag it to
~/Library/QuickLook. You may need to create that folder if it doesn't already exist.Alternative, if you use Homebrew-Cask, install with
brew cask install qlcolorcode.